Expandable thermoplastic polymers and among these, in particular, expandable polystyrene (EPS), are products which have been known and used for a long time for preparing expanded articles which can be adopted in various applicative areas, among which, one of the most important is thermal insulation.
These expanded products are obtained by first swelling the polymer granules, in a closed environment, impregnated with an expandable fluid, for example an aliphatic hydrocarbon such as pentane or hexane, and then molding/sintering the swollen particles, charged inside a mould, by the contemporaneous effect of pressure and temperature. The swelling of the particles, as also their sintering, is generally effected with vapour, or another gas, maintained at a temperature slightly higher than the glass transition temperature (Tg) of the polymer.
A particular applicative field, for example of expanded polystyrene, is that of thermal insulation in the building industry where it is generally used in the form of flat sheets. The flat sheets of expanded polystyrene are normally used with a density of about 30 g/l as the thermal conductivity of the polymer has a minimum at these values. It is not advantageous to fall below this limit, even if this is technically possible, as it causes a drastic increase in the thermal conductivity of the sheet which must be compensated by an increase in its thickness. In order to avoid this drawback, suggestions have been made to fill the polymer with athermanous materials such as graphite, carbon black or aluminium. Athermanous materials are in fact capable of interacting with the radioactive flow (infrared radiation), reducing its transmission and thus increasing the thermal insulation of the expanded materials in which they are contained.
The best performances of thermal insulation allow a significant reduction in the density of the expanded article or thickness of the same without reducing the overall thermal resistance value.
European patent 620,246, for example, describes a process for preparing granules of expandable polystyrene containing an athermanous material, for example carbon black, distributed on the surface or, alternatively, incorporated inside the particle itself.
The use of carbon black has long been known as a filler or pigment, or else as a nucleating agent (see, for example, Chem. Abstr., 1987, “Carbon Black Containing Polystyrene Beads”).
Among the various types of carbon black, the most important are carbon black from oil combustion (“petroleum black”), carbon black from gas combustion, carbon black from acetylene, lamp black, channel black, thermal black and electrically conductive carbon black. International patent application WO 1997/45477 describes compositions based on expandable polystyrene comprising a styrene polymer, from 0.05 to 25% of carbon black of the lamp black type, and from 0.6 to 5% of a brominated additive to make the product fireproof.
Depending on the manufacturing process, these carbon blacks have diameters which range from 10 nm to 1,000 nm approximately, and have very different specific surfaces (from 10 to 2,000 m2/g). These differences lead to different blockage capacities of the infrared rays. International patent application WO 2006/61571 describes compositions based on expandable polystyrene comprising a styrene polymer having a weight average molecular weight Mw of 150,000-450,000, from 2 to 20% by weight of an expanding agent and from 0.05 to less than 1% of carbon black, with a surface area ranging from 550 to 1,600 m2/g.
It is known that graphite can also be effectively used as a blackbody (as described, for example, in JP 63-183941, WO 04/022636, WO 96/34039). Its use as attenuating agent of infrared radiation in polymeric foams is, however, more recent. Patent application JP 63-183941 is among the first to propose the use of some additives, active in blocking infrared rays in wavelengths ranging from 6 to 14 microns, and therefore obtaining thermally insulating thermoplastic resins capable of permanently sustaining a low thermal conductivity. Among all additives, graphite is preferred.
Patent DE 9305431U describes a method for producing expanded moulded products having a density of less than 20 kg/m3 and a reduced thermal conductivity. This result is reached by incorporating an athermanous material, such as graphite and also carbon black, in the rigid polystyrene foam.
International patent application WO 98/51735 describes expandable polystyrene particulates containing 0.05 to 25% by weight of synthetic or natural graphite particles, homogeneously distributed in the polystyrene matrix. The graphite preferably has an average diameter ranging from 1 to 50 μm, an apparent density ranging from 100 to 500 g/l and a surface area ranging from 5 to 20 m2/g.